We characterize cell motion in experiments and show that the transition to collective motion in colonies of gliding bacterial cells confined to a monolayer appears through the organization of cells into larger moving clusters. Collective motion by non-equilibrium cluster formation is detected for a critical cell packing fraction around 17%. This transition is characterized by a scale-free power-law cluster size distribution, with an exponent 0.88±0.07, and the appearance of giant number fluctuations. Our findings are in quantitative agreement with simulations of self-propelled rods. This suggests that the interplay of self-propulsion of bacteria and the rod-shape of bacteria is sufficient to induce collective motion.
@article{arxiv.1302.0311,
title = {Collective motion and nonequilibrium cluster formation in colonies of gliding bacteria},
author = {Fernando Peruani and Joern Starruss and Vladimir Jakovljevic and Lotte Sogaard-Andersen and Andreas Deutsch and Markus Bar},
journal= {arXiv preprint arXiv:1302.0311},
year = {2013}
}